Project 4 Project Summary Skeletal metastasis is a final and inevitable consequence of advanced prostate cancer. Previous studies aimed to improve understanding of cancer and bone focused primarily on interactions of tumor cells and bone cells. Other critical cellular interactions have been understudied. Myeloid cells play intriguing roles infiltrating tumor tissue and promoting tumor growth; however little is known of the mechanisms of action of myeloid cells in the bone marrow relative to tumor occupation in bone. Tumor cells inevitably encounter these cells during their trajectory from the primary lesion, during dormancy, and in the frank metastatic lesion. Most of the interest in myeloid cells in cancer has centered on the cytokines they produce and/or immune reactions that they orchestrate. Macrophages mediate a specialized process of apoptotic cell phagocytosis termed efferocytosis. The distinct signaling pathways that macrophages use to identify apoptotic cells for efferocytosis are emerging as therapeutic targets for autoimmune conditions and atherosclerosis. Preliminary investigations from the project laboratory demonstrate that macrophage efferocytosis induces polarization towards M2-type tumor- promoting cells with activation of the Stat3-RelA signaling axis. Furthermore, M2-type macrophage depletion with a novel therapeutic, significantly reduces experimental tumor progression in the bone microenvironment. This proposal sets a goal to delineate pathways relative to prostate cancer cell apoptosis and tumor growth during skeletal metastasis using in vitro strategies, novel animal models, as well as human cells and tissues to validate the translation and clinical potential for targeted therapies. The overall hypothesis is that bone marrow macrophages support prostate cancer growth in bone via phagocytosis/efferocytosis of apoptotic tumor cells. Three aims will address the intriguing question as to how myeloid/macrophage cells drive tumor establishment in the bone marrow. The first will identify the phagocytic/efferocytotic macrophage in the tumor microenvironment of skeletal metastases and determine its ability to support tumor growth. The second will determine the mechanisms of action of milk fat globule EGF-8 (MFGE8), a mediator of efferocytosis, in supporting tumor growth. The third aim will elucidate targetable mechanisms of macrophage efferocytosis and prostate cancer tumor progression using strategies that directly translate to clinical therapeutic potential